Node configuration

Environment variables:

• NUVEX_NODE_HEALTH_BIND, default 127.0.0.1:8081
• NUVEX_LOG_FORMAT, json or text
• NUVEX_SOLANA_RPC_URL, optional and unused
• NUVEX_NODE_KEY_PATH, optional and unread

There is no default cluster URL. Debug formatting redacts the URL and the key path.